Heart failure (HF) and related complications constitute major health burden in developed countries with an estimated prevalence of 5,700,000 in the United States alone (Roger, V. L. et al., Circulation, 125(1):e2-e220 (2012)). Despite considerable advances in recent two decades, the prognosis remains very poor, with survival rates of only ˜50% within 5-years of diagnosis (Roger, V. L. et al., JAMA, 292(3):344-350 (2004)). In addition to poor survival, the impaired quality of life and recurrent hospitalizations constitute clear unmet medical need for development of novel treatment options.
HF is a clinical syndrome characterized by the inability of the heart to deliver sufficient supply of blood and oxygen to meet the metabolic demands of organs in the body. Main symptoms associated with HF include shortness of breath due to pulmonary edema, fatigue, reduced tolerance to exercise and lower extremity edemas. The etiology of HF is highly complex with multiple associated risk factors and potential causes.
Among the leading causes of HF are coronary artery disease and cardiac ischemia, acute myocardial infarction, intrinsic cardiomyopathies and chronic uncontrolled hypertension. HF can develop either acutely (functional impairment post myocardial infarction) or as a chronic condition, characterized by long-term maladaptive cardiac tissue remodeling, hypertrophy and cardiac dysfunction (for example due to uncontrolled long-term hypertension). According to the diagnostic criteria and type of ventricular dysfunction, HF is classified to two major groups, HF with “reduced ejection fraction” (HFrEF) or HF with “preserved ejection fraction” (HFpEF). Both types are associated with similar signs and symptoms, but differ in the type of ventricular functional impairment (Borlaug, B. A. et al., Eur. Heart J., 32(6):670-679 (2011)).
APJ receptor (APLNR) and its endogenous peptidic ligand apelin have been implicated as important modulators of cardiovascular function and candidates for therapeutic intervention in HF (for review see Japp, A. G. et al., Biochem. Pharmacol., 75(10):1882-1892 (2008)).
Accumulated evidence from preclinical disease models and human heart failure patients have implicated apelin and APJ agonism as beneficial in the setting of HF. Mice lacking Apelin or APJ gene have impaired myocyte contractility (Charo, D. N. et al., Am. J. Physiol. Heart Circ. Physiol., 297(5):H1904-H1913 (2009)). Apelin knockout (KO) mice develop progressive cardiac dysfunction with aging and are more susceptible to HF in the model of trans-aortic constriction (TAC) (Kuba, K. et al., Circ. Res., 101(4):e32-42 (2007)). The functional impairment in chronic HF is a result of prolonged demand on the heart and is associated with maladaptive cardiac remodeling, manifested by the cardiac hypertrophy, increased inflammation and interstitial fibrosis which eventually lead to decrease in cardiac performance.
Acute administration of apelin increases cardiac output in rodents under normal conditions and also in models of heart failure (Berry, M. F., Circulation, 110(11 Suppl. 1):II187-II193 (2004)). Increased cardiac output is a result of direct augmentation of cardiac contractility and reduced peripheral vascular resistance in the arterial and venous beds (Ashley, E. A., Cardiovasc. Res., 65(1):73-82 (2005)). Reduction in the vascular resistance leads to lower pre-load and after-load on the heart and thus lesser work load (Cheng, X. et al., Eur. J. Pharmacol., 470(3):171-175 (2003)). Similar to rodent studies, acute infusion of apelin to healthy human subjects and patients with heart failure produces similar hemodynamic responses with increased cardiac output and increased vasodilatory response in peripheral and coronary arteries (Japp, A. G. et al., Circulation, 121(16):1818-1827 (2010)).
The mechanisms underlying inotropic action of apelin are not well understood, but appear to be distinct from clinically used β1-adrenergic agonists (dobutamine) due to lack of increase in heart rate. The vasodilatory action of apelin is primarily mediated via endothelial nitric oxide synthase pathways (Tatemoto, K., Regul. Pept., 99(2-3):87-92 (2001)). Apelin is induced under hypoxic conditions, promotes angiogenesis and has been shown to limit the infarct size in ischemia-reperfusion models (Simpkin, J. C., Basic Res. Cardiol., 102(6):518-528 (2007)).
In addition to aforementioned studies evaluating acute administration of apelin, several studies have clearly demonstrated beneficial effects of prolonged administration of apelin in a number of chronic rodent models of HF, including the angiotensin II model, TAC model and rat Dahl salt-sensitive model (Siddiquee, K. et al., J. Hypertens., 29(4):724-731 (2011); Scimia, M. C. et al., Nature, 488(7411):394-398 (2012); Koguchi, W. et al., Circ. J., 76(1):137-144 (2012)). In these studies, prolonged apelin infusion reduced cardiac hypertrophy and cardiac fibrosis, and was associated with improvement in cardiac performance.
Genetic evidence is also emerging that polymorphisms in the APJ gene are associated with slower progression of HF (Sarzani, R. et al., J. Card. Fail., 13(7):521-529 (2007)). Importantly, while expression of APJ and apelin can be reduced or vary considerably with HF progression, the cardiovascular hemodynamic effects of apelin are sustained in patients with developed HF and receiving standard of care therapy (Japp, A. G. et al., Circulation, 121(16):1818-1827 (2010)).
In summary, there is a significant amount of evidence to indicate that APJ receptor agonism plays a cardioprotective role in HF and would be of potential benefit to HF patients. Apelin's very short half life in circulation limits its therapeutic utility, and consequently, there is a need for APJ receptor agonists with improved pharmacokinetic and signaling profile while maintaining or enhancing the beneficial effects of endogenous APJ agonist apelin.